diff --git a/Makefile b/Makefile
index fb50c57e4f..a43b8ee067 100644
--- a/Makefile
+++ b/Makefile
@@ -1216,7 +1216,9 @@ LIB_OBJS += object.o
 LIB_OBJS += odb.o
 LIB_OBJS += odb/source.o
 LIB_OBJS += odb/source-files.o
+LIB_OBJS += odb/source-inmemory.o
 LIB_OBJS += odb/streaming.o
+LIB_OBJS += odb/transaction.o
 LIB_OBJS += oid-array.o
 LIB_OBJS += oidmap.o
 LIB_OBJS += oidset.o
@@ -1525,6 +1527,7 @@ CLAR_TEST_SUITES += u-hash
 CLAR_TEST_SUITES += u-hashmap
 CLAR_TEST_SUITES += u-list-objects-filter-options
 CLAR_TEST_SUITES += u-mem-pool
+CLAR_TEST_SUITES += u-odb-inmemory
 CLAR_TEST_SUITES += u-oid-array
 CLAR_TEST_SUITES += u-oidmap
 CLAR_TEST_SUITES += u-oidtree
diff --git a/builtin/add.c b/builtin/add.c
index 7737ab878b..c859f66519 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -16,6 +16,7 @@
 #include "run-command.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/transaction.h"
 #include "parse-options.h"
 #include "path.h"
 #include "preload-index.h"
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 12452aff60..59e9b8711e 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -9,6 +9,8 @@
 #include "hex.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/streaming.h"
+#include "odb/transaction.h"
 #include "object.h"
 #include "delta.h"
 #include "pack.h"
@@ -358,24 +360,21 @@ static void unpack_non_delta_entry(enum object_type type, unsigned long size,
 
 struct input_zstream_data {
 	git_zstream *zstream;
-	unsigned char buf[8192];
 	int status;
 };
 
-static const void *feed_input_zstream(struct odb_write_stream *in_stream,
-				      unsigned long *readlen)
+static ssize_t feed_input_zstream(struct odb_write_stream *in_stream,
+				  unsigned char *buf, size_t buf_len)
 {
 	struct input_zstream_data *data = in_stream->data;
 	git_zstream *zstream = data->zstream;
 	void *in = fill(1);
 
-	if (in_stream->is_finished) {
-		*readlen = 0;
-		return NULL;
-	}
+	if (in_stream->is_finished)
+		return 0;
 
-	zstream->next_out = data->buf;
-	zstream->avail_out = sizeof(data->buf);
+	zstream->next_out = buf;
+	zstream->avail_out = buf_len;
 	zstream->next_in = in;
 	zstream->avail_in = len;
 
@@ -383,9 +382,7 @@ static const void *feed_input_zstream(struct odb_write_stream *in_stream,
 
 	in_stream->is_finished = data->status != Z_OK;
 	use(len - zstream->avail_in);
-	*readlen = sizeof(data->buf) - zstream->avail_out;
-
-	return data->buf;
+	return buf_len - zstream->avail_out;
 }
 
 static void stream_blob(unsigned long size, unsigned nr)
diff --git a/builtin/update-index.c b/builtin/update-index.c
index 8a5907767b..bcc43852ef 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -19,6 +19,7 @@
 #include "tree-walk.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/transaction.h"
 #include "refs.h"
 #include "resolve-undo.h"
 #include "parse-options.h"
diff --git a/cache-tree.c b/cache-tree.c
index 2b636eb3f8..184f7e2635 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -10,6 +10,7 @@
 #include "cache-tree.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/transaction.h"
 #include "read-cache-ll.h"
 #include "replace-object.h"
 #include "repository.h"
diff --git a/cbtree.c b/cbtree.c
index 4ab794bddc..8f5edbb80a 100644
--- a/cbtree.c
+++ b/cbtree.c
@@ -7,6 +7,11 @@
 #include "git-compat-util.h"
 #include "cbtree.h"
 
+static inline uint8_t *cb_node_key(struct cb_tree *t, struct cb_node *node)
+{
+	return (uint8_t *) node + t->key_offset;
+}
+
 static struct cb_node *cb_node_of(const void *p)
 {
 	return (struct cb_node *)((uintptr_t)p - 1);
@@ -33,6 +38,7 @@ struct cb_node *cb_insert(struct cb_tree *t, struct cb_node *node, size_t klen)
 	uint8_t c;
 	int newdirection;
 	struct cb_node **wherep, *p;
+	uint8_t *node_key, *p_key;
 
 	assert(!((uintptr_t)node & 1)); /* allocations must be aligned */
 
@@ -41,23 +47,26 @@ struct cb_node *cb_insert(struct cb_tree *t, struct cb_node *node, size_t klen)
 		return NULL;	/* success */
 	}
 
+	node_key = cb_node_key(t, node);
+
 	/* see if a node already exists */
-	p = cb_internal_best_match(t->root, node->k, klen);
+	p = cb_internal_best_match(t->root, node_key, klen);
+	p_key = cb_node_key(t, p);
 
 	/* find first differing byte */
 	for (newbyte = 0; newbyte < klen; newbyte++) {
-		if (p->k[newbyte] != node->k[newbyte])
+		if (p_key[newbyte] != node_key[newbyte])
 			goto different_byte_found;
 	}
 	return p;	/* element exists, let user deal with it */
 
 different_byte_found:
-	newotherbits = p->k[newbyte] ^ node->k[newbyte];
+	newotherbits = p_key[newbyte] ^ node_key[newbyte];
 	newotherbits |= newotherbits >> 1;
 	newotherbits |= newotherbits >> 2;
 	newotherbits |= newotherbits >> 4;
 	newotherbits = (newotherbits & ~(newotherbits >> 1)) ^ 255;
-	c = p->k[newbyte];
+	c = p_key[newbyte];
 	newdirection = (1 + (newotherbits | c)) >> 8;
 
 	node->byte = newbyte;
@@ -78,7 +87,7 @@ different_byte_found:
 			break;
 		if (q->byte == newbyte && q->otherbits > newotherbits)
 			break;
-		c = q->byte < klen ? node->k[q->byte] : 0;
+		c = q->byte < klen ? node_key[q->byte] : 0;
 		direction = (1 + (q->otherbits | c)) >> 8;
 		wherep = q->child + direction;
 	}
@@ -93,7 +102,7 @@ struct cb_node *cb_lookup(struct cb_tree *t, const uint8_t *k, size_t klen)
 {
 	struct cb_node *p = cb_internal_best_match(t->root, k, klen);
 
-	return p && !memcmp(p->k, k, klen) ? p : NULL;
+	return p && !memcmp(cb_node_key(t, p), k, klen) ? p : NULL;
 }
 
 static int cb_descend(struct cb_node *p, cb_iter fn, void *arg)
@@ -115,6 +124,7 @@ int cb_each(struct cb_tree *t, const uint8_t *kpfx, size_t klen,
 	struct cb_node *p = t->root;
 	struct cb_node *top = p;
 	size_t i = 0;
+	uint8_t *p_key;
 
 	if (!p)
 		return 0; /* empty tree */
@@ -130,8 +140,9 @@ int cb_each(struct cb_tree *t, const uint8_t *kpfx, size_t klen,
 			top = p;
 	}
 
+	p_key = cb_node_key(t, p);
 	for (i = 0; i < klen; i++) {
-		if (p->k[i] != kpfx[i])
+		if (p_key[i] != kpfx[i])
 			return 0; /* "best" match failed */
 	}
 
diff --git a/cbtree.h b/cbtree.h
index c374b1b3db..4647d4a32f 100644
--- a/cbtree.h
+++ b/cbtree.h
@@ -6,9 +6,9 @@
  *
  * This is adapted to store arbitrary data (not just NUL-terminated C strings
  * and allocates no memory internally.  The user needs to allocate
- * "struct cb_node" and fill cb_node.k[] with arbitrary match data
- * for memcmp.
- * If "klen" is variable, then it should be embedded into "c_node.k[]"
+ * "struct cb_node" and provide `key_offset` to indicate where the key can be
+ * found relative to the `struct cb_node` for memcmp.
+ * If "klen" is variable, then it should be embedded into the key.
  * Recursion is bound by the maximum value of "klen" used.
  */
 #ifndef CBTREE_H
@@ -23,18 +23,19 @@ struct cb_node {
 	 */
 	uint32_t byte;
 	uint8_t otherbits;
-	uint8_t k[FLEX_ARRAY]; /* arbitrary data, unaligned */
 };
 
 struct cb_tree {
 	struct cb_node *root;
+	ptrdiff_t key_offset;
 };
 
-#define CBTREE_INIT { 0 }
-
-static inline void cb_init(struct cb_tree *t)
+static inline void cb_init(struct cb_tree *t,
+			   ptrdiff_t key_offset)
 {
-	struct cb_tree blank = CBTREE_INIT;
+	struct cb_tree blank = {
+		.key_offset = key_offset,
+	};
 	memcpy(t, &blank, sizeof(*t));
 }
 
diff --git a/loose.c b/loose.c
index 07333be696..f7a3dd1a72 100644
--- a/loose.c
+++ b/loose.c
@@ -57,7 +57,7 @@ static int insert_loose_map(struct odb_source *source,
 	inserted |= insert_oid_pair(map->to_compat, oid, compat_oid);
 	inserted |= insert_oid_pair(map->to_storage, compat_oid, oid);
 	if (inserted)
-		oidtree_insert(files->loose->cache, compat_oid);
+		oidtree_insert(files->loose->cache, compat_oid, NULL);
 
 	return inserted;
 }
diff --git a/meson.build b/meson.build
index 052c81f288..664d831329 100644
--- a/meson.build
+++ b/meson.build
@@ -404,7 +404,9 @@ libgit_sources = [
   'odb.c',
   'odb/source.c',
   'odb/source-files.c',
+  'odb/source-inmemory.c',
   'odb/streaming.c',
+  'odb/transaction.c',
   'oid-array.c',
   'oidmap.c',
   'oidset.c',
diff --git a/object-file.c b/object-file.c
index 0be2981c7a..90f995d000 100644
--- a/object-file.c
+++ b/object-file.c
@@ -21,6 +21,7 @@
 #include "object-file.h"
 #include "odb.h"
 #include "odb/streaming.h"
+#include "odb/transaction.h"
 #include "oidtree.h"
 #include "pack.h"
 #include "packfile.h"
@@ -1068,6 +1069,7 @@ int odb_source_loose_write_stream(struct odb_source *source,
 	struct git_hash_ctx c, compat_c;
 	struct strbuf tmp_file = STRBUF_INIT;
 	struct strbuf filename = STRBUF_INIT;
+	unsigned char buf[8192];
 	int dirlen;
 	char hdr[MAX_HEADER_LEN];
 	int hdrlen;
@@ -1100,9 +1102,17 @@ int odb_source_loose_write_stream(struct odb_source *source,
 		unsigned char *in0 = stream.next_in;
 
 		if (!stream.avail_in && !in_stream->is_finished) {
-			const void *in = in_stream->read(in_stream, &stream.avail_in);
-			stream.next_in = (void *)in;
-			in0 = (unsigned char *)in;
+			ssize_t read_len = odb_write_stream_read(in_stream, buf,
+								 sizeof(buf));
+			if (read_len < 0) {
+				close(fd);
+				err = -1;
+				goto cleanup;
+			}
+
+			stream.avail_in = read_len;
+			stream.next_in = buf;
+			in0 = buf;
 			/* All data has been read. */
 			if (in_stream->is_finished)
 				flush = 1;
@@ -1392,11 +1402,10 @@ static int already_written(struct odb_transaction_files *transaction,
 }
 
 /* Lazily create backing packfile for the state */
-static void prepare_packfile_transaction(struct odb_transaction_files *transaction,
-					 unsigned flags)
+static void prepare_packfile_transaction(struct odb_transaction_files *transaction)
 {
 	struct transaction_packfile *state = &transaction->packfile;
-	if (!(flags & INDEX_WRITE_OBJECT) || state->f)
+	if (state->f)
 		return;
 
 	state->f = create_tmp_packfile(transaction->base.source->odb->repo,
@@ -1409,33 +1418,53 @@ static void prepare_packfile_transaction(struct odb_transaction_files *transacti
 		die_errno("unable to write pack header");
 }
 
+static int hash_blob_stream(struct odb_write_stream *stream,
+			    const struct git_hash_algo *hash_algo,
+			    struct object_id *result_oid, size_t size)
+{
+	unsigned char buf[16384];
+	struct git_hash_ctx ctx;
+	unsigned header_len;
+	size_t bytes_hashed = 0;
+
+	header_len = format_object_header((char *)buf, sizeof(buf),
+					  OBJ_BLOB, size);
+	hash_algo->init_fn(&ctx);
+	git_hash_update(&ctx, buf, header_len);
+
+	while (!stream->is_finished) {
+		ssize_t read_result = odb_write_stream_read(stream, buf,
+							    sizeof(buf));
+
+		if (read_result < 0)
+			return -1;
+
+		git_hash_update(&ctx, buf, read_result);
+		bytes_hashed += read_result;
+	}
+
+	if (bytes_hashed != size)
+		return -1;
+
+	git_hash_final_oid(result_oid, &ctx);
+
+	return 0;
+}
+
 /*
- * Read the contents from fd for size bytes, streaming it to the
- * packfile in state while updating the hash in ctx. Signal a failure
- * by returning a negative value when the resulting pack would exceed
- * the pack size limit and this is not the first object in the pack,
- * so that the caller can discard what we wrote from the current pack
- * by truncating it and opening a new one. The caller will then call
- * us again after rewinding the input fd.
- *
- * The already_hashed_to pointer is kept untouched by the caller to
- * make sure we do not hash the same byte when we are called
- * again. This way, the caller does not have to checkpoint its hash
- * status before calling us just in case we ask it to call us again
- * with a new pack.
+ * Read the contents from the stream provided, streaming it to the
+ * packfile in state while updating the hash in ctx.
  */
-static int stream_blob_to_pack(struct transaction_packfile *state,
-			       struct git_hash_ctx *ctx, off_t *already_hashed_to,
-			       int fd, size_t size, const char *path,
-			       unsigned flags)
+static void stream_blob_to_pack(struct transaction_packfile *state,
+				struct git_hash_ctx *ctx, size_t size,
+				struct odb_write_stream *stream)
 {
 	git_zstream s;
 	unsigned char ibuf[16384];
 	unsigned char obuf[16384];
 	unsigned hdrlen;
 	int status = Z_OK;
-	int write_object = (flags & INDEX_WRITE_OBJECT);
-	off_t offset = 0;
+	size_t bytes_read = 0;
 
 	git_deflate_init(&s, pack_compression_level);
 
@@ -1444,45 +1473,27 @@ static int stream_blob_to_pack(struct transaction_packfile *state,
 	s.avail_out = sizeof(obuf) - hdrlen;
 
 	while (status != Z_STREAM_END) {
-		if (size && !s.avail_in) {
-			size_t rsize = size < sizeof(ibuf) ? size : sizeof(ibuf);
-			ssize_t read_result = read_in_full(fd, ibuf, rsize);
-			if (read_result < 0)
-				die_errno("failed to read from '%s'", path);
-			if ((size_t)read_result != rsize)
-				die("failed to read %u bytes from '%s'",
-				    (unsigned)rsize, path);
-			offset += rsize;
-			if (*already_hashed_to < offset) {
-				size_t hsize = offset - *already_hashed_to;
-				if (rsize < hsize)
-					hsize = rsize;
-				if (hsize)
-					git_hash_update(ctx, ibuf, hsize);
-				*already_hashed_to = offset;
-			}
+		if (!stream->is_finished && !s.avail_in) {
+			ssize_t rsize = odb_write_stream_read(stream, ibuf,
+							      sizeof(ibuf));
+
+			if (rsize < 0)
+				die("failed to read blob data");
+
+			git_hash_update(ctx, ibuf, rsize);
+
 			s.next_in = ibuf;
 			s.avail_in = rsize;
-			size -= rsize;
+			bytes_read += rsize;
 		}
 
-		status = git_deflate(&s, size ? 0 : Z_FINISH);
+		status = git_deflate(&s, stream->is_finished ? Z_FINISH : 0);
 
 		if (!s.avail_out || status == Z_STREAM_END) {
-			if (write_object) {
-				size_t written = s.next_out - obuf;
+			size_t written = s.next_out - obuf;
 
-				/* would we bust the size limit? */
-				if (state->nr_written &&
-				    pack_size_limit_cfg &&
-				    pack_size_limit_cfg < state->offset + written) {
-					git_deflate_abort(&s);
-					return -1;
-				}
-
-				hashwrite(state->f, obuf, written);
-				state->offset += written;
-			}
+			hashwrite(state->f, obuf, written);
+			state->offset += written;
 			s.next_out = obuf;
 			s.avail_out = sizeof(obuf);
 		}
@@ -1496,8 +1507,12 @@ static int stream_blob_to_pack(struct transaction_packfile *state,
 			die("unexpected deflate failure: %d", status);
 		}
 	}
+
+	if (bytes_read != size)
+		die("read %" PRIuMAX " bytes of blob data, but expected %" PRIuMAX " bytes",
+		    (uintmax_t)bytes_read, (uintmax_t)size);
+
 	git_deflate_end(&s);
-	return 0;
 }
 
 static void flush_packfile_transaction(struct odb_transaction_files *transaction)
@@ -1568,64 +1583,49 @@ clear_exit:
  * binary blobs, they generally do not want to get any conversion, and
  * callers should avoid this code path when filters are requested.
  */
-static int index_blob_packfile_transaction(struct odb_transaction_files *transaction,
-					   struct object_id *result_oid, int fd,
-					   size_t size, const char *path,
-					   unsigned flags)
+static int odb_transaction_files_write_object_stream(struct odb_transaction *base,
+						     struct odb_write_stream *stream,
+						     size_t size,
+						     struct object_id *result_oid)
 {
+	struct odb_transaction_files *transaction = container_of(base,
+								 struct odb_transaction_files,
+								 base);
 	struct transaction_packfile *state = &transaction->packfile;
-	off_t seekback, already_hashed_to;
 	struct git_hash_ctx ctx;
 	unsigned char obuf[16384];
 	unsigned header_len;
 	struct hashfile_checkpoint checkpoint;
-	struct pack_idx_entry *idx = NULL;
-
-	seekback = lseek(fd, 0, SEEK_CUR);
-	if (seekback == (off_t)-1)
-		return error("cannot find the current offset");
+	struct pack_idx_entry *idx;
 
 	header_len = format_object_header((char *)obuf, sizeof(obuf),
 					  OBJ_BLOB, size);
 	transaction->base.source->odb->repo->hash_algo->init_fn(&ctx);
 	git_hash_update(&ctx, obuf, header_len);
 
-	/* Note: idx is non-NULL when we are writing */
-	if ((flags & INDEX_WRITE_OBJECT) != 0) {
-		CALLOC_ARRAY(idx, 1);
-
-		prepare_packfile_transaction(transaction, flags);
-		hashfile_checkpoint_init(state->f, &checkpoint);
-	}
-
-	already_hashed_to = 0;
-
-	while (1) {
-		prepare_packfile_transaction(transaction, flags);
-		if (idx) {
-			hashfile_checkpoint(state->f, &checkpoint);
-			idx->offset = state->offset;
-			crc32_begin(state->f);
-		}
-		if (!stream_blob_to_pack(state, &ctx, &already_hashed_to,
-					 fd, size, path, flags))
-			break;
-		/*
-		 * Writing this object to the current pack will make
-		 * it too big; we need to truncate it, start a new
-		 * pack, and write into it.
-		 */
-		if (!idx)
-			BUG("should not happen");
-		hashfile_truncate(state->f, &checkpoint);
-		state->offset = checkpoint.offset;
+	/*
+	 * If writing another object to the packfile could result in it
+	 * exceeding the configured size limit, flush the current packfile
+	 * transaction.
+	 *
+	 * Note that this uses the inflated object size as an approximation.
+	 * Blob objects written in this manner are not delta-compressed, so
+	 * the difference between the inflated and on-disk size is limited
+	 * to zlib compression and is sufficient for this check.
+	 */
+	if (state->nr_written && pack_size_limit_cfg &&
+	    pack_size_limit_cfg < state->offset + size)
 		flush_packfile_transaction(transaction);
-		if (lseek(fd, seekback, SEEK_SET) == (off_t)-1)
-			return error("cannot seek back");
-	}
+
+	CALLOC_ARRAY(idx, 1);
+	prepare_packfile_transaction(transaction);
+	hashfile_checkpoint_init(state->f, &checkpoint);
+
+	hashfile_checkpoint(state->f, &checkpoint);
+	idx->offset = state->offset;
+	crc32_begin(state->f);
+	stream_blob_to_pack(state, &ctx, size, stream);
 	git_hash_final_oid(result_oid, &ctx);
-	if (!idx)
-		return 0;
 
 	idx->crc32 = crc32_end(state->f);
 	if (already_written(transaction, result_oid)) {
@@ -1642,34 +1642,6 @@ static int index_blob_packfile_transaction(struct odb_transaction_files *transac
 	return 0;
 }
 
-static int hash_blob_stream(const struct git_hash_algo *hash_algo,
-			    struct object_id *result_oid, int fd, size_t size)
-{
-	unsigned char buf[16384];
-	struct git_hash_ctx ctx;
-	unsigned header_len;
-
-	header_len = format_object_header((char *)buf, sizeof(buf),
-					  OBJ_BLOB, size);
-	hash_algo->init_fn(&ctx);
-	git_hash_update(&ctx, buf, header_len);
-
-	while (size) {
-		size_t rsize = size < sizeof(buf) ? size : sizeof(buf);
-		ssize_t read_result = read_in_full(fd, buf, rsize);
-
-		if ((read_result < 0) || ((size_t)read_result != rsize))
-			return -1;
-
-		git_hash_update(&ctx, buf, rsize);
-		size -= read_result;
-	}
-
-	git_hash_final_oid(result_oid, &ctx);
-
-	return 0;
-}
-
 int index_fd(struct index_state *istate, struct object_id *oid,
 	     int fd, struct stat *st,
 	     enum object_type type, const char *path, unsigned flags)
@@ -1691,23 +1663,25 @@ int index_fd(struct index_state *istate, struct object_id *oid,
 		ret = index_core(istate, oid, fd, xsize_t(st->st_size),
 				 type, path, flags);
 	} else {
+		struct odb_write_stream stream;
+		odb_write_stream_from_fd(&stream, fd, xsize_t(st->st_size));
+
 		if (flags & INDEX_WRITE_OBJECT) {
 			struct object_database *odb = the_repository->objects;
-			struct odb_transaction_files *files_transaction;
-			struct odb_transaction *transaction;
+			struct odb_transaction *transaction = odb_transaction_begin(odb);
 
-			transaction = odb_transaction_begin(odb);
-			files_transaction = container_of(odb->transaction,
-							 struct odb_transaction_files,
-							 base);
-			ret = index_blob_packfile_transaction(files_transaction, oid, fd,
-							      xsize_t(st->st_size),
-							      path, flags);
+			ret = odb_transaction_write_object_stream(odb->transaction,
+								  &stream,
+								  xsize_t(st->st_size),
+								  oid);
 			odb_transaction_commit(transaction);
 		} else {
-			ret = hash_blob_stream(the_repository->hash_algo, oid,
-					       fd, xsize_t(st->st_size));
+			ret = hash_blob_stream(&stream,
+					       the_repository->hash_algo, oid,
+					       xsize_t(st->st_size));
 		}
+
+		odb_write_stream_release(&stream);
 	}
 
 	close(fd);
@@ -1884,6 +1858,7 @@ static int for_each_object_wrapper_cb(const struct object_id *oid,
 }
 
 static int for_each_prefixed_object_wrapper_cb(const struct object_id *oid,
+					       void *node_data UNUSED,
 					       void *cb_data)
 {
 	struct for_each_object_wrapper_data *data = cb_data;
@@ -2029,7 +2004,7 @@ static int append_loose_object(const struct object_id *oid,
 			       const char *path UNUSED,
 			       void *data)
 {
-	oidtree_insert(data, oid);
+	oidtree_insert(data, oid, NULL);
 	return 0;
 }
 
@@ -2225,6 +2200,7 @@ struct odb_transaction *odb_transaction_files_begin(struct odb_source *source)
 	transaction = xcalloc(1, sizeof(*transaction));
 	transaction->base.source = source;
 	transaction->base.commit = odb_transaction_files_commit;
+	transaction->base.write_object_stream = odb_transaction_files_write_object_stream;
 
 	return &transaction->base;
 }
diff --git a/odb.c b/odb.c
index 9b28fe25ef..965ef68e4e 100644
--- a/odb.c
+++ b/odb.c
@@ -14,6 +14,7 @@
 #include "object-file.h"
 #include "object-name.h"
 #include "odb.h"
+#include "odb/source-inmemory.h"
 #include "packfile.h"
 #include "path.h"
 #include "promisor-remote.h"
@@ -31,40 +32,6 @@
 KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct odb_source *, 1, fspathhash, fspatheq)
 
-/*
- * This is meant to hold a *small* number of objects that you would
- * want odb_read_object() to be able to return, but yet you do not want
- * to write them into the object store (e.g. a browse-only
- * application).
- */
-struct cached_object_entry {
-	struct object_id oid;
-	struct cached_object {
-		enum object_type type;
-		const void *buf;
-		unsigned long size;
-	} value;
-};
-
-static const struct cached_object *find_cached_object(struct object_database *object_store,
-						      const struct object_id *oid)
-{
-	static const struct cached_object empty_tree = {
-		.type = OBJ_TREE,
-		.buf = "",
-	};
-	const struct cached_object_entry *co = object_store->cached_objects;
-
-	for (size_t i = 0; i < object_store->cached_object_nr; i++, co++)
-		if (oideq(&co->oid, oid))
-			return &co->value;
-
-	if (oid->algo && oideq(oid, hash_algos[oid->algo].empty_tree))
-		return &empty_tree;
-
-	return NULL;
-}
-
 int odb_mkstemp(struct object_database *odb,
 		struct strbuf *temp_filename, const char *pattern)
 {
@@ -584,7 +551,6 @@ static int do_oid_object_info_extended(struct object_database *odb,
 				       const struct object_id *oid,
 				       struct object_info *oi, unsigned flags)
 {
-	const struct cached_object *co;
 	const struct object_id *real = oid;
 	int already_retried = 0;
 
@@ -594,25 +560,8 @@ static int do_oid_object_info_extended(struct object_database *odb,
 	if (is_null_oid(real))
 		return -1;
 
-	co = find_cached_object(odb, real);
-	if (co) {
-		if (oi) {
-			if (oi->typep)
-				*(oi->typep) = co->type;
-			if (oi->sizep)
-				*(oi->sizep) = co->size;
-			if (oi->disk_sizep)
-				*(oi->disk_sizep) = 0;
-			if (oi->delta_base_oid)
-				oidclr(oi->delta_base_oid, odb->repo->hash_algo);
-			if (oi->contentp)
-				*oi->contentp = xmemdupz(co->buf, co->size);
-			if (oi->mtimep)
-				*oi->mtimep = 0;
-			oi->whence = OI_CACHED;
-		}
+	if (!odb_source_read_object_info(odb->inmemory_objects, oid, oi, flags))
 		return 0;
-	}
 
 	odb_prepare_alternates(odb);
 
@@ -784,24 +733,12 @@ int odb_pretend_object(struct object_database *odb,
 		       void *buf, unsigned long len, enum object_type type,
 		       struct object_id *oid)
 {
-	struct cached_object_entry *co;
-	char *co_buf;
-
 	hash_object_file(odb->repo->hash_algo, buf, len, type, oid);
-	if (odb_has_object(odb, oid, 0) ||
-	    find_cached_object(odb, oid))
+	if (odb_has_object(odb, oid, 0))
 		return 0;
 
-	ALLOC_GROW(odb->cached_objects,
-		   odb->cached_object_nr + 1, odb->cached_object_alloc);
-	co = &odb->cached_objects[odb->cached_object_nr++];
-	co->value.size = len;
-	co->value.type = type;
-	co_buf = xmalloc(len);
-	memcpy(co_buf, buf, len);
-	co->value.buf = co_buf;
-	oidcpy(&co->oid, oid);
-	return 0;
+	return odb_source_write_object(odb->inmemory_objects,
+				       buf, len, type, oid, NULL, 0);
 }
 
 void *odb_read_object(struct object_database *odb,
@@ -1083,6 +1020,7 @@ struct object_database *odb_new(struct repository *repo,
 	o->sources = odb_source_new(o, primary_source, true);
 	o->sources_tail = &o->sources->next;
 	o->alternate_db = xstrdup_or_null(secondary_sources);
+	o->inmemory_objects = &odb_source_inmemory_new(o)->base;
 
 	free(to_free);
 
@@ -1106,6 +1044,10 @@ static void odb_free_sources(struct object_database *o)
 		odb_source_free(o->sources);
 		o->sources = next;
 	}
+
+	odb_source_free(o->inmemory_objects);
+	o->inmemory_objects = NULL;
+
 	kh_destroy_odb_path_map(o->source_by_path);
 	o->source_by_path = NULL;
 }
@@ -1123,10 +1065,6 @@ void odb_free(struct object_database *o)
 	odb_close(o);
 	odb_free_sources(o);
 
-	for (size_t i = 0; i < o->cached_object_nr; i++)
-		free((char *) o->cached_objects[i].value.buf);
-	free(o->cached_objects);
-
 	string_list_clear(&o->submodule_source_paths, 0);
 
 	free(o);
@@ -1154,28 +1092,3 @@ void odb_reprepare(struct object_database *o)
 
 	obj_read_unlock();
 }
-
-struct odb_transaction *odb_transaction_begin(struct object_database *odb)
-{
-	if (odb->transaction)
-		return NULL;
-
-	odb->transaction = odb_transaction_files_begin(odb->sources);
-
-	return odb->transaction;
-}
-
-void odb_transaction_commit(struct odb_transaction *transaction)
-{
-	if (!transaction)
-		return;
-
-	/*
-	 * Ensure the transaction ending matches the pending transaction.
-	 */
-	ASSERT(transaction == transaction->source->odb->transaction);
-
-	transaction->commit(transaction);
-	transaction->source->odb->transaction = NULL;
-	free(transaction);
-}
diff --git a/odb.h b/odb.h
index 3a711f6547..73553ed5a7 100644
--- a/odb.h
+++ b/odb.h
@@ -8,6 +8,7 @@
 #include "thread-utils.h"
 
 struct cached_object_entry;
+struct odb_source_inmemory;
 struct packed_git;
 struct repository;
 struct strbuf;
@@ -29,24 +30,6 @@ extern int fetch_if_missing;
  */
 char *compute_alternate_path(const char *path, struct strbuf *err);
 
-/*
- * A transaction may be started for an object database prior to writing new
- * objects via odb_transaction_begin(). These objects are not committed until
- * odb_transaction_commit() is invoked. Only a single transaction may be pending
- * at a time.
- *
- * Each ODB source is expected to implement its own transaction handling.
- */
-struct odb_transaction;
-typedef void (*odb_transaction_commit_fn)(struct odb_transaction *transaction);
-struct odb_transaction {
-	/* The ODB source the transaction is opened against. */
-	struct odb_source *source;
-
-	/* The ODB source specific callback invoked to commit a transaction. */
-	odb_transaction_commit_fn commit;
-};
-
 /*
  * The object database encapsulates access to objects in a repository. It
  * manages one or more sources that store the actual objects which are
@@ -98,8 +81,7 @@ struct object_database {
 	 * to write them into the object store (e.g. a browse-only
 	 * application).
 	 */
-	struct cached_object_entry *cached_objects;
-	size_t cached_object_nr, cached_object_alloc;
+	struct odb_source *inmemory_objects;
 
 	/*
 	 * A fast, rough count of the number of objects in the repository.
@@ -148,19 +130,6 @@ void odb_close(struct object_database *o);
  */
 void odb_reprepare(struct object_database *o);
 
-/*
- * Starts an ODB transaction. Subsequent objects are written to the transaction
- * and not committed until odb_transaction_commit() is invoked on the
- * transaction. If the ODB already has a pending transaction, NULL is returned.
- */
-struct odb_transaction *odb_transaction_begin(struct object_database *odb);
-
-/*
- * Commits an ODB transaction making the written objects visible. If the
- * specified transaction is NULL, the function is a no-op.
- */
-void odb_transaction_commit(struct odb_transaction *transaction);
-
 /*
  * Find source by its object directory path. Returns a `NULL` pointer in case
  * the source could not be found.
@@ -593,11 +562,7 @@ static inline int odb_write_object(struct object_database *odb,
 	return odb_write_object_ext(odb, buf, len, type, oid, NULL, 0);
 }
 
-struct odb_write_stream {
-	const void *(*read)(struct odb_write_stream *, unsigned long *len);
-	void *data;
-	int is_finished;
-};
+struct odb_write_stream;
 
 int odb_write_object_stream(struct object_database *odb,
 			    struct odb_write_stream *stream, size_t len,
diff --git a/odb/source-inmemory.c b/odb/source-inmemory.c
new file mode 100644
index 0000000000..e004566d76
--- /dev/null
+++ b/odb/source-inmemory.c
@@ -0,0 +1,382 @@
+#include "git-compat-util.h"
+#include "object-file.h"
+#include "odb.h"
+#include "odb/source-inmemory.h"
+#include "odb/streaming.h"
+#include "oidtree.h"
+#include "repository.h"
+
+struct inmemory_object {
+	enum object_type type;
+	const void *buf;
+	unsigned long size;
+};
+
+static const struct inmemory_object *find_cached_object(struct odb_source_inmemory *source,
+							const struct object_id *oid)
+{
+	static const struct inmemory_object empty_tree = {
+		.type = OBJ_TREE,
+		.buf = "",
+	};
+	const struct inmemory_object *object;
+
+	if (source->objects) {
+		object = oidtree_get(source->objects, oid);
+		if (object)
+			return object;
+	}
+
+	if (oid->algo && oideq(oid, hash_algos[oid->algo].empty_tree))
+		return &empty_tree;
+
+	return NULL;
+}
+
+static void populate_object_info(struct odb_source_inmemory *source,
+				 struct object_info *oi,
+				 const struct inmemory_object *object)
+{
+	if (!oi)
+		return;
+
+	if (oi->typep)
+		*(oi->typep) = object->type;
+	if (oi->sizep)
+		*(oi->sizep) = object->size;
+	if (oi->disk_sizep)
+		*(oi->disk_sizep) = 0;
+	if (oi->delta_base_oid)
+		oidclr(oi->delta_base_oid, source->base.odb->repo->hash_algo);
+	if (oi->contentp)
+		*oi->contentp = xmemdupz(object->buf, object->size);
+	if (oi->mtimep)
+		*oi->mtimep = 0;
+	oi->whence = OI_CACHED;
+}
+
+static int odb_source_inmemory_read_object_info(struct odb_source *source,
+						const struct object_id *oid,
+						struct object_info *oi,
+						enum object_info_flags flags UNUSED)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+	const struct inmemory_object *object;
+
+	object = find_cached_object(inmemory, oid);
+	if (!object)
+		return -1;
+
+	populate_object_info(inmemory, oi, object);
+	return 0;
+}
+
+struct odb_read_stream_inmemory {
+	struct odb_read_stream base;
+	const unsigned char *buf;
+	size_t offset;
+};
+
+static ssize_t odb_read_stream_inmemory_read(struct odb_read_stream *stream,
+					     char *buf, size_t buf_len)
+{
+	struct odb_read_stream_inmemory *inmemory =
+		container_of(stream, struct odb_read_stream_inmemory, base);
+	size_t bytes = buf_len;
+
+	if (buf_len > inmemory->base.size - inmemory->offset)
+		bytes = inmemory->base.size - inmemory->offset;
+
+	memcpy(buf, inmemory->buf + inmemory->offset, bytes);
+	inmemory->offset += bytes;
+
+	return bytes;
+}
+
+static int odb_read_stream_inmemory_close(struct odb_read_stream *stream UNUSED)
+{
+	return 0;
+}
+
+static int odb_source_inmemory_read_object_stream(struct odb_read_stream **out,
+						  struct odb_source *source,
+						  const struct object_id *oid)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+	struct odb_read_stream_inmemory *stream;
+	const struct inmemory_object *object;
+
+	object = find_cached_object(inmemory, oid);
+	if (!object)
+		return -1;
+
+	CALLOC_ARRAY(stream, 1);
+	stream->base.read = odb_read_stream_inmemory_read;
+	stream->base.close = odb_read_stream_inmemory_close;
+	stream->base.size = object->size;
+	stream->base.type = object->type;
+	stream->buf = object->buf;
+
+	*out = &stream->base;
+	return 0;
+}
+
+struct odb_source_inmemory_for_each_object_data {
+	struct odb_source_inmemory *inmemory;
+	const struct object_info *request;
+	odb_for_each_object_cb cb;
+	void *cb_data;
+};
+
+static int odb_source_inmemory_for_each_object_cb(const struct object_id *oid,
+						  void *node_data, void *cb_data)
+{
+	struct odb_source_inmemory_for_each_object_data *data = cb_data;
+	struct inmemory_object *object = node_data;
+
+	if (data->request) {
+		struct object_info oi = *data->request;
+		populate_object_info(data->inmemory, &oi, object);
+		return data->cb(oid, &oi, data->cb_data);
+	} else {
+		return data->cb(oid, NULL, data->cb_data);
+	}
+}
+
+static int odb_source_inmemory_for_each_object(struct odb_source *source,
+					       const struct object_info *request,
+					       odb_for_each_object_cb cb,
+					       void *cb_data,
+					       const struct odb_for_each_object_options *opts)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+	struct odb_source_inmemory_for_each_object_data payload = {
+		.inmemory = inmemory,
+		.request = request,
+		.cb = cb,
+		.cb_data = cb_data,
+	};
+	struct object_id null_oid = { 0 };
+
+	if ((opts->flags & ODB_FOR_EACH_OBJECT_PROMISOR_ONLY) ||
+	    (opts->flags & ODB_FOR_EACH_OBJECT_LOCAL_ONLY && !source->local))
+		return 0;
+	if (!inmemory->objects)
+		return 0;
+
+	return oidtree_each(inmemory->objects,
+			    opts->prefix ? opts->prefix : &null_oid, opts->prefix_hex_len,
+			    odb_source_inmemory_for_each_object_cb, &payload);
+}
+
+struct find_abbrev_len_data {
+	const struct object_id *oid;
+	unsigned len;
+};
+
+static int find_abbrev_len_cb(const struct object_id *oid,
+			      struct object_info *oi UNUSED,
+			      void *cb_data)
+{
+	struct find_abbrev_len_data *data = cb_data;
+	unsigned len = oid_common_prefix_hexlen(oid, data->oid);
+	if (len != hash_algos[oid->algo].hexsz && len >= data->len)
+		data->len = len + 1;
+	return 0;
+}
+
+static int odb_source_inmemory_find_abbrev_len(struct odb_source *source,
+					       const struct object_id *oid,
+					       unsigned min_len,
+					       unsigned *out)
+{
+	struct odb_for_each_object_options opts = {
+		.prefix = oid,
+		.prefix_hex_len = min_len,
+	};
+	struct find_abbrev_len_data data = {
+		.oid = oid,
+		.len = min_len,
+	};
+	int ret;
+
+	ret = odb_source_inmemory_for_each_object(source, NULL, find_abbrev_len_cb,
+						  &data, &opts);
+	*out = data.len;
+
+	return ret;
+}
+
+static int count_objects_cb(const struct object_id *oid UNUSED,
+			    struct object_info *oi UNUSED,
+			    void *cb_data)
+{
+	unsigned long *counter = cb_data;
+	(*counter)++;
+	return 0;
+}
+
+static int odb_source_inmemory_count_objects(struct odb_source *source,
+					     enum odb_count_objects_flags flags UNUSED,
+					     unsigned long *out)
+{
+	struct odb_for_each_object_options opts = { 0 };
+	*out = 0;
+	return odb_source_inmemory_for_each_object(source, NULL, count_objects_cb,
+						   out, &opts);
+}
+
+static int odb_source_inmemory_write_object(struct odb_source *source,
+					    const void *buf, unsigned long len,
+					    enum object_type type,
+					    struct object_id *oid,
+					    struct object_id *compat_oid UNUSED,
+					    enum odb_write_object_flags flags UNUSED)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+	struct inmemory_object *object;
+
+	hash_object_file(source->odb->repo->hash_algo, buf, len, type, oid);
+
+	if (!inmemory->objects) {
+		CALLOC_ARRAY(inmemory->objects, 1);
+		oidtree_init(inmemory->objects);
+	} else if (oidtree_contains(inmemory->objects, oid)) {
+		return 0;
+	}
+
+	CALLOC_ARRAY(object, 1);
+	object->size = len;
+	object->type = type;
+	object->buf = xmemdupz(buf, len);
+
+	oidtree_insert(inmemory->objects, oid, object);
+
+	return 0;
+}
+
+static int odb_source_inmemory_write_object_stream(struct odb_source *source,
+						   struct odb_write_stream *stream,
+						   size_t len,
+						   struct object_id *oid)
+{
+	char buf[16384];
+	size_t total_read = 0;
+	char *data;
+	int ret;
+
+	CALLOC_ARRAY(data, len);
+	while (!stream->is_finished) {
+		ssize_t bytes_read;
+
+		bytes_read = odb_write_stream_read(stream, buf, sizeof(buf));
+		if (total_read + bytes_read > len) {
+			ret = error("object stream yielded more bytes than expected");
+			goto out;
+		}
+
+		memcpy(data + total_read, buf, bytes_read);
+		total_read += bytes_read;
+	}
+
+	if (total_read != len) {
+		ret = error("object stream yielded less bytes than expected");
+		goto out;
+	}
+
+	ret = odb_source_inmemory_write_object(source, data, len, OBJ_BLOB, oid,
+					       NULL, 0);
+	if (ret < 0)
+		goto out;
+
+out:
+	free(data);
+	return ret;
+}
+
+static int odb_source_inmemory_freshen_object(struct odb_source *source,
+					      const struct object_id *oid)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+	if (find_cached_object(inmemory, oid))
+		return 1;
+	return 0;
+}
+
+static int odb_source_inmemory_begin_transaction(struct odb_source *source UNUSED,
+						 struct odb_transaction **out UNUSED)
+{
+	return error("in-memory source does not support transactions");
+}
+
+static int odb_source_inmemory_read_alternates(struct odb_source *source UNUSED,
+					       struct strvec *out UNUSED)
+{
+	return 0;
+}
+
+static int odb_source_inmemory_write_alternate(struct odb_source *source UNUSED,
+					       const char *alternate UNUSED)
+{
+	return error("in-memory source does not support alternates");
+}
+
+static void odb_source_inmemory_close(struct odb_source *source UNUSED)
+{
+}
+
+static void odb_source_inmemory_reprepare(struct odb_source *source UNUSED)
+{
+}
+
+static int inmemory_object_free(const struct object_id *oid UNUSED,
+				void *node_data,
+				void *cb_data UNUSED)
+{
+	struct inmemory_object *object = node_data;
+	free((void *) object->buf);
+	free(object);
+	return 0;
+}
+
+static void odb_source_inmemory_free(struct odb_source *source)
+{
+	struct odb_source_inmemory *inmemory = odb_source_inmemory_downcast(source);
+
+	if (inmemory->objects) {
+		struct object_id null_oid = { 0 };
+
+		oidtree_each(inmemory->objects, &null_oid, 0,
+			     inmemory_object_free, NULL);
+		oidtree_clear(inmemory->objects);
+		free(inmemory->objects);
+	}
+
+	free(inmemory->base.path);
+	free(inmemory);
+}
+
+struct odb_source_inmemory *odb_source_inmemory_new(struct object_database *odb)
+{
+	struct odb_source_inmemory *source;
+
+	CALLOC_ARRAY(source, 1);
+	odb_source_init(&source->base, odb, ODB_SOURCE_INMEMORY, "source", false);
+
+	source->base.free = odb_source_inmemory_free;
+	source->base.close = odb_source_inmemory_close;
+	source->base.reprepare = odb_source_inmemory_reprepare;
+	source->base.read_object_info = odb_source_inmemory_read_object_info;
+	source->base.read_object_stream = odb_source_inmemory_read_object_stream;
+	source->base.for_each_object = odb_source_inmemory_for_each_object;
+	source->base.find_abbrev_len = odb_source_inmemory_find_abbrev_len;
+	source->base.count_objects = odb_source_inmemory_count_objects;
+	source->base.write_object = odb_source_inmemory_write_object;
+	source->base.write_object_stream = odb_source_inmemory_write_object_stream;
+	source->base.freshen_object = odb_source_inmemory_freshen_object;
+	source->base.begin_transaction = odb_source_inmemory_begin_transaction;
+	source->base.read_alternates = odb_source_inmemory_read_alternates;
+	source->base.write_alternate = odb_source_inmemory_write_alternate;
+
+	return source;
+}
diff --git a/odb/source-inmemory.h b/odb/source-inmemory.h
new file mode 100644
index 0000000000..a88fc2e320
--- /dev/null
+++ b/odb/source-inmemory.h
@@ -0,0 +1,33 @@
+#ifndef ODB_SOURCE_INMEMORY_H
+#define ODB_SOURCE_INMEMORY_H
+
+#include "odb/source.h"
+
+struct oidtree;
+
+/*
+ * An in-memory source that you can write objects to that shall be made
+ * available for reading, but that shouldn't ever be persisted to disk. Note
+ * that any objects written to this source will be stored in memory, so the
+ * number of objects you can store is limited by available system memory.
+ */
+struct odb_source_inmemory {
+	struct odb_source base;
+	struct oidtree *objects;
+};
+
+/* Create a new in-memory object database source. */
+struct odb_source_inmemory *odb_source_inmemory_new(struct object_database *odb);
+
+/*
+ * Cast the given object database source to the in-memory backend. This will
+ * cause a BUG in case the source doesn't use this backend.
+ */
+static inline struct odb_source_inmemory *odb_source_inmemory_downcast(struct odb_source *source)
+{
+	if (source->type != ODB_SOURCE_INMEMORY)
+		BUG("trying to downcast source of type '%d' to in-memory", source->type);
+	return container_of(source, struct odb_source_inmemory, base);
+}
+
+#endif
diff --git a/odb/source.h b/odb/source.h
index f706e0608a..0a440884e4 100644
--- a/odb/source.h
+++ b/odb/source.h
@@ -13,6 +13,9 @@ enum odb_source_type {
 
 	/* The "files" backend that uses loose objects and packfiles. */
 	ODB_SOURCE_FILES,
+
+	/* The "in-memory" backend that stores objects in memory. */
+	ODB_SOURCE_INMEMORY,
 };
 
 struct object_id;
diff --git a/odb/streaming.c b/odb/streaming.c
index af2adf5ce7..7602a8d5d8 100644
--- a/odb/streaming.c
+++ b/odb/streaming.c
@@ -243,6 +243,16 @@ struct odb_read_stream *odb_read_stream_open(struct object_database *odb,
 	return st;
 }
 
+ssize_t odb_write_stream_read(struct odb_write_stream *st, void *buf, size_t sz)
+{
+	return st->read(st, buf, sz);
+}
+
+void odb_write_stream_release(struct odb_write_stream *st)
+{
+	free(st->data);
+}
+
 int odb_stream_blob_to_fd(struct object_database *odb,
 			  int fd,
 			  const struct object_id *oid,
@@ -298,3 +308,44 @@ int odb_stream_blob_to_fd(struct object_database *odb,
 	odb_read_stream_close(st);
 	return result;
 }
+
+struct read_object_fd_data {
+	int fd;
+	size_t remaining;
+};
+
+static ssize_t read_object_fd(struct odb_write_stream *stream,
+			      unsigned char *buf, size_t len)
+{
+	struct read_object_fd_data *data = stream->data;
+	ssize_t read_result;
+	size_t count;
+
+	if (stream->is_finished)
+		return 0;
+
+	count = data->remaining < len ? data->remaining : len;
+	read_result = read_in_full(data->fd, buf, count);
+	if (read_result < 0 || (size_t)read_result != count)
+		return -1;
+
+	data->remaining -= count;
+	if (!data->remaining)
+		stream->is_finished = 1;
+
+	return read_result;
+}
+
+void odb_write_stream_from_fd(struct odb_write_stream *stream, int fd,
+			      size_t size)
+{
+	struct read_object_fd_data *data;
+
+	CALLOC_ARRAY(data, 1);
+	data->fd = fd;
+	data->remaining = size;
+
+	stream->data = data;
+	stream->read = read_object_fd;
+	stream->is_finished = 0;
+}
diff --git a/odb/streaming.h b/odb/streaming.h
index 517e2ea2d3..c023671780 100644
--- a/odb/streaming.h
+++ b/odb/streaming.h
@@ -5,6 +5,7 @@
 #define STREAMING_H 1
 
 #include "object.h"
+#include "odb.h"
 
 struct object_database;
 struct odb_read_stream;
@@ -47,6 +48,29 @@ int odb_read_stream_close(struct odb_read_stream *stream);
  */
 ssize_t odb_read_stream_read(struct odb_read_stream *stream, void *buf, size_t len);
 
+/*
+ * A stream that provides an object to be written to the object database without
+ * loading all of it into memory.
+ */
+struct odb_write_stream {
+	ssize_t (*read)(struct odb_write_stream *, unsigned char *, size_t);
+	void *data;
+	int is_finished;
+};
+
+/*
+ * Read data from the stream into the buffer. Returns 0 when finished and the
+ * number of bytes read on success. Returns a negative error code in case
+ * reading from the stream fails.
+ */
+ssize_t odb_write_stream_read(struct odb_write_stream *stream, void *buf,
+			      size_t len);
+
+/*
+ * Releases memory allocated for underlying stream data.
+ */
+void odb_write_stream_release(struct odb_write_stream *stream);
+
 /*
  * Look up the object by its ID and write the full contents to the file
  * descriptor. The object must be a blob, or the function will fail. When
@@ -64,4 +88,10 @@ int odb_stream_blob_to_fd(struct object_database *odb,
 			  struct stream_filter *filter,
 			  int can_seek);
 
+/*
+ * Sets up an ODB write stream that reads from an fd.
+ */
+void odb_write_stream_from_fd(struct odb_write_stream *stream, int fd,
+			      size_t size);
+
 #endif /* STREAMING_H */
diff --git a/odb/transaction.c b/odb/transaction.c
new file mode 100644
index 0000000000..b16e07aebf
--- /dev/null
+++ b/odb/transaction.c
@@ -0,0 +1,35 @@
+#include "git-compat-util.h"
+#include "odb/source.h"
+#include "odb/transaction.h"
+
+struct odb_transaction *odb_transaction_begin(struct object_database *odb)
+{
+	if (odb->transaction)
+		return NULL;
+
+	odb_source_begin_transaction(odb->sources, &odb->transaction);
+
+	return odb->transaction;
+}
+
+void odb_transaction_commit(struct odb_transaction *transaction)
+{
+	if (!transaction)
+		return;
+
+	/*
+	 * Ensure the transaction ending matches the pending transaction.
+	 */
+	ASSERT(transaction == transaction->source->odb->transaction);
+
+	transaction->commit(transaction);
+	transaction->source->odb->transaction = NULL;
+	free(transaction);
+}
+
+int odb_transaction_write_object_stream(struct odb_transaction *transaction,
+					struct odb_write_stream *stream,
+					size_t len, struct object_id *oid)
+{
+	return transaction->write_object_stream(transaction, stream, len, oid);
+}
diff --git a/odb/transaction.h b/odb/transaction.h
new file mode 100644
index 0000000000..854fda06f5
--- /dev/null
+++ b/odb/transaction.h
@@ -0,0 +1,57 @@
+#ifndef ODB_TRANSACTION_H
+#define ODB_TRANSACTION_H
+
+#include "odb.h"
+#include "odb/source.h"
+
+/*
+ * A transaction may be started for an object database prior to writing new
+ * objects via odb_transaction_begin(). These objects are not committed until
+ * odb_transaction_commit() is invoked. Only a single transaction may be pending
+ * at a time.
+ *
+ * Each ODB source is expected to implement its own transaction handling.
+ */
+struct odb_transaction {
+	/* The ODB source the transaction is opened against. */
+	struct odb_source *source;
+
+	/* The ODB source specific callback invoked to commit a transaction. */
+	void (*commit)(struct odb_transaction *transaction);
+
+	/*
+	 * This callback is expected to write the given object stream into
+	 * the ODB transaction. Note that for now, only blobs support streaming.
+	 *
+	 * The resulting object ID shall be written into the out pointer. The
+	 * callback is expected to return 0 on success, a negative error code
+	 * otherwise.
+	 */
+	int (*write_object_stream)(struct odb_transaction *transaction,
+				   struct odb_write_stream *stream, size_t len,
+				   struct object_id *oid);
+};
+
+/*
+ * Starts an ODB transaction. Subsequent objects are written to the transaction
+ * and not committed until odb_transaction_commit() is invoked on the
+ * transaction. If the ODB already has a pending transaction, NULL is returned.
+ */
+struct odb_transaction *odb_transaction_begin(struct object_database *odb);
+
+/*
+ * Commits an ODB transaction making the written objects visible. If the
+ * specified transaction is NULL, the function is a no-op.
+ */
+void odb_transaction_commit(struct odb_transaction *transaction);
+
+/*
+ * Writes the object in the provided stream into the transaction. The resulting
+ * object ID is written into the out pointer. Returns 0 on success, a negative
+ * error code otherwise.
+ */
+int odb_transaction_write_object_stream(struct odb_transaction *transaction,
+					struct odb_write_stream *stream,
+					size_t len, struct object_id *oid);
+
+#endif
diff --git a/oidtree.c b/oidtree.c
index ab9fe7ec7a..e43f18026e 100644
--- a/oidtree.c
+++ b/oidtree.c
@@ -6,9 +6,15 @@
 #include "oidtree.h"
 #include "hash.h"
 
+struct oidtree_node {
+	struct cb_node base;
+	struct object_id key;
+	void *data;
+};
+
 void oidtree_init(struct oidtree *ot)
 {
-	cb_init(&ot->tree);
+	cb_init(&ot->tree, offsetof(struct oidtree_node, key));
 	mem_pool_init(&ot->mem_pool, 0);
 }
 
@@ -20,22 +26,22 @@ void oidtree_clear(struct oidtree *ot)
 	}
 }
 
-void oidtree_insert(struct oidtree *ot, const struct object_id *oid)
+struct oidtree_data {
+	struct object_id oid;
+};
+
+void oidtree_insert(struct oidtree *ot, const struct object_id *oid,
+		    void *data)
 {
-	struct cb_node *on;
-	struct object_id k;
+	struct oidtree_node *on;
+	struct cb_node *node;
 
 	if (!oid->algo)
 		BUG("oidtree_insert requires oid->algo");
 
-	on = mem_pool_alloc(&ot->mem_pool, sizeof(*on) + sizeof(*oid));
-
-	/*
-	 * Clear the padding and copy the result in separate steps to
-	 * respect the 4-byte alignment needed by struct object_id.
-	 */
-	oidcpy(&k, oid);
-	memcpy(on->k, &k, sizeof(k));
+	on = mem_pool_alloc(&ot->mem_pool, sizeof(*on));
+	oidcpy(&on->key, oid);
+	on->data = data;
 
 	/*
 	 * n.b. Current callers won't get us duplicates, here.  If a
@@ -43,13 +49,19 @@ void oidtree_insert(struct oidtree *ot, const struct object_id *oid)
 	 * that won't be freed until oidtree_clear.  Currently it's not
 	 * worth maintaining a free list
 	 */
-	cb_insert(&ot->tree, on, sizeof(*oid));
+	node = cb_insert(&ot->tree, &on->base, sizeof(*oid));
+	if (node) {
+		struct oidtree_node *preexisting = container_of(node, struct oidtree_node, base);
+		preexisting->data = data;
+	}
 }
 
-bool oidtree_contains(struct oidtree *ot, const struct object_id *oid)
+static struct oidtree_node *oidtree_lookup(struct oidtree *ot,
+					   const struct object_id *oid)
 {
 	struct object_id k;
 	size_t klen = sizeof(k);
+	struct cb_node *node;
 
 	oidcpy(&k, oid);
 
@@ -60,7 +72,20 @@ bool oidtree_contains(struct oidtree *ot, const struct object_id *oid)
 	klen += BUILD_ASSERT_OR_ZERO(offsetof(struct object_id, hash) <
 				offsetof(struct object_id, algo));
 
-	return !!cb_lookup(&ot->tree, (const uint8_t *)&k, klen);
+	node = cb_lookup(&ot->tree, (const uint8_t *)&k, klen);
+	return node ? container_of(node, struct oidtree_node, base) : NULL;
+}
+
+bool oidtree_contains(struct oidtree *ot, const struct object_id *oid)
+{
+	struct oidtree_node *node = oidtree_lookup(ot, oid);
+	return node ? 1 : 0;
+}
+
+void *oidtree_get(struct oidtree *ot, const struct object_id *oid)
+{
+	struct oidtree_node *node = oidtree_lookup(ot, oid);
+	return node ? node->data : NULL;
 }
 
 struct oidtree_each_data {
@@ -73,21 +98,18 @@ struct oidtree_each_data {
 
 static int iter(struct cb_node *n, void *cb_data)
 {
+	struct oidtree_node *node = container_of(n, struct oidtree_node, base);
 	struct oidtree_each_data *data = cb_data;
-	struct object_id k;
 
-	/* Copy to provide 4-byte alignment needed by struct object_id. */
-	memcpy(&k, n->k, sizeof(k));
-
-	if (data->algo != GIT_HASH_UNKNOWN && data->algo != k.algo)
+	if (data->algo != GIT_HASH_UNKNOWN && data->algo != node->key.algo)
 		return 0;
 
 	if (data->last_nibble_at) {
-		if ((k.hash[*data->last_nibble_at] ^ data->last_byte) & 0xf0)
+		if ((node->key.hash[*data->last_nibble_at] ^ data->last_byte) & 0xf0)
 			return 0;
 	}
 
-	return data->cb(&k, data->cb_data);
+	return data->cb(&node->key, node->data, data->cb_data);
 }
 
 int oidtree_each(struct oidtree *ot, const struct object_id *prefix,
diff --git a/oidtree.h b/oidtree.h
index 2b7bad2e60..baa5a436ea 100644
--- a/oidtree.h
+++ b/oidtree.h
@@ -29,18 +29,26 @@ void oidtree_init(struct oidtree *ot);
  */
 void oidtree_clear(struct oidtree *ot);
 
-/* Insert the object ID into the tree. */
-void oidtree_insert(struct oidtree *ot, const struct object_id *oid);
+/*
+ * Insert the object ID into the tree and store the given pointer alongside
+ * with it. The data pointer of any preexisting entry will be overwritten.
+ */
+void oidtree_insert(struct oidtree *ot, const struct object_id *oid,
+		    void *data);
 
 /* Check whether the tree contains the given object ID. */
 bool oidtree_contains(struct oidtree *ot, const struct object_id *oid);
 
+/* Get the payload stored with the given object ID. */
+void *oidtree_get(struct oidtree *ot, const struct object_id *oid);
+
 /*
  * Callback function used for `oidtree_each()`. Returning a non-zero exit code
  * will cause iteration to stop. The exit code will be propagated to the caller
  * of `oidtree_each()`.
  */
 typedef int (*oidtree_each_cb)(const struct object_id *oid,
+			       void *node_data,
 			       void *cb_data);
 
 /*
diff --git a/read-cache.c b/read-cache.c
index 38a04b8de3..21829102ae 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -20,6 +20,7 @@
 #include "dir.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/transaction.h"
 #include "oid-array.h"
 #include "tree.h"
 #include "commit.h"
diff --git a/t/meson.build b/t/meson.build
index fd955f44ef..2ad6a801cd 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -6,6 +6,7 @@ clar_test_suites = [
   'unit-tests/u-hashmap.c',
   'unit-tests/u-list-objects-filter-options.c',
   'unit-tests/u-mem-pool.c',
+  'unit-tests/u-odb-inmemory.c',
   'unit-tests/u-oid-array.c',
   'unit-tests/u-oidmap.c',
   'unit-tests/u-oidtree.c',
diff --git a/t/unit-tests/u-odb-inmemory.c b/t/unit-tests/u-odb-inmemory.c
new file mode 100644
index 0000000000..482502ef4b
--- /dev/null
+++ b/t/unit-tests/u-odb-inmemory.c
@@ -0,0 +1,313 @@
+#include "unit-test.h"
+#include "hex.h"
+#include "odb/source-inmemory.h"
+#include "odb/streaming.h"
+#include "oidset.h"
+#include "repository.h"
+#include "strbuf.h"
+
+#define RANDOM_OID "da39a3ee5e6b4b0d3255bfef95601890afd80709"
+#define FOOBAR_OID "f6ea0495187600e7b2288c8ac19c5886383a4632"
+
+static struct repository repo = {
+	.hash_algo = &hash_algos[GIT_HASH_SHA1],
+};
+static struct object_database *odb;
+
+static void cl_assert_object_info(struct odb_source_inmemory *source,
+				  const struct object_id *oid,
+				  enum object_type expected_type,
+				  const char *expected_content)
+{
+	enum object_type actual_type;
+	unsigned long actual_size;
+	void *actual_content;
+	struct object_info oi = {
+		.typep = &actual_type,
+		.sizep = &actual_size,
+		.contentp = &actual_content,
+	};
+
+	cl_must_pass(odb_source_read_object_info(&source->base, oid, &oi, 0));
+	cl_assert_equal_u(actual_size, strlen(expected_content));
+	cl_assert_equal_u(actual_type, expected_type);
+	cl_assert_equal_s((char *) actual_content, expected_content);
+
+	free(actual_content);
+}
+
+void test_odb_inmemory__initialize(void)
+{
+	odb = odb_new(&repo, "", "");
+}
+
+void test_odb_inmemory__cleanup(void)
+{
+	odb_free(odb);
+}
+
+void test_odb_inmemory__new(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	cl_assert_equal_i(source->base.type, ODB_SOURCE_INMEMORY);
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__read_missing_object(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct object_id oid;
+	const char *end;
+
+	cl_must_pass(parse_oid_hex_algop(RANDOM_OID, &oid, &end, repo.hash_algo));
+	cl_must_fail(odb_source_read_object_info(&source->base, &oid, NULL, 0));
+
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__read_empty_tree(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	cl_assert_object_info(source, repo.hash_algo->empty_tree, OBJ_TREE, "");
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__read_written_object(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	const char data[] = "foobar";
+	struct object_id written_oid;
+
+	cl_must_pass(odb_source_write_object(&source->base, data, strlen(data),
+					     OBJ_BLOB, &written_oid, NULL, 0));
+	cl_assert_equal_s(oid_to_hex(&written_oid), FOOBAR_OID);
+	cl_assert_object_info(source, &written_oid, OBJ_BLOB, "foobar");
+
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__read_stream_object(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct odb_read_stream *stream;
+	struct object_id written_oid;
+	const char data[] = "foobar";
+	char buf[3] = { 0 };
+
+	cl_must_pass(odb_source_write_object(&source->base, data, strlen(data),
+					     OBJ_BLOB, &written_oid, NULL, 0));
+
+	cl_must_pass(odb_source_read_object_stream(&stream, &source->base,
+						   &written_oid));
+	cl_assert_equal_i(stream->type, OBJ_BLOB);
+	cl_assert_equal_u(stream->size, 6);
+
+	cl_assert_equal_i(odb_read_stream_read(stream, buf, 2), 2);
+	cl_assert_equal_s(buf, "fo");
+	cl_assert_equal_i(odb_read_stream_read(stream, buf, 2), 2);
+	cl_assert_equal_s(buf, "ob");
+	cl_assert_equal_i(odb_read_stream_read(stream, buf, 2), 2);
+	cl_assert_equal_s(buf, "ar");
+	cl_assert_equal_i(odb_read_stream_read(stream, buf, 2), 0);
+
+	odb_read_stream_close(stream);
+	odb_source_free(&source->base);
+}
+
+static int add_one_object(const struct object_id *oid,
+			  struct object_info *oi UNUSED,
+			  void *payload)
+{
+	struct oidset *actual_oids = payload;
+	cl_must_pass(oidset_insert(actual_oids, oid));
+	return 0;
+}
+
+void test_odb_inmemory__for_each_object(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct odb_for_each_object_options opts = { 0 };
+	struct oidset expected_oids = OIDSET_INIT;
+	struct oidset actual_oids = OIDSET_INIT;
+	struct strbuf buf = STRBUF_INIT;
+
+	cl_must_pass(odb_source_for_each_object(&source->base, NULL,
+						add_one_object, &actual_oids, &opts));
+	cl_assert_equal_u(oidset_size(&actual_oids), 0);
+
+	for (int i = 0; i < 10; i++) {
+		struct object_id written_oid;
+
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "%d", i);
+
+		cl_must_pass(odb_source_write_object(&source->base, buf.buf, buf.len,
+						     OBJ_BLOB, &written_oid, NULL, 0));
+		cl_must_pass(oidset_insert(&expected_oids, &written_oid));
+	}
+
+	cl_must_pass(odb_source_for_each_object(&source->base, NULL,
+						add_one_object, &actual_oids, &opts));
+	cl_assert_equal_b(oidset_equal(&expected_oids, &actual_oids), true);
+
+	odb_source_free(&source->base);
+	oidset_clear(&expected_oids);
+	oidset_clear(&actual_oids);
+	strbuf_release(&buf);
+}
+
+static int abort_after_two_objects(const struct object_id *oid UNUSED,
+				   struct object_info *oi UNUSED,
+				   void *payload)
+{
+	unsigned *counter = payload;
+	(*counter)++;
+	if (*counter == 2)
+		return 123;
+	return 0;
+}
+
+void test_odb_inmemory__for_each_object_can_abort_iteration(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct odb_for_each_object_options opts = { 0 };
+	struct object_id written_oid;
+	unsigned counter = 0;
+
+	cl_must_pass(odb_source_write_object(&source->base, "1", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+	cl_must_pass(odb_source_write_object(&source->base, "2", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+	cl_must_pass(odb_source_write_object(&source->base, "3", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+
+	cl_assert_equal_i(odb_source_for_each_object(&source->base, NULL,
+						     abort_after_two_objects,
+						     &counter, &opts),
+			  123);
+	cl_assert_equal_u(counter, 2);
+
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__count_objects(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct object_id written_oid;
+	unsigned long count;
+
+	cl_must_pass(odb_source_count_objects(&source->base, 0, &count));
+	cl_assert_equal_u(count, 0);
+
+	cl_must_pass(odb_source_write_object(&source->base, "1", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+	cl_must_pass(odb_source_write_object(&source->base, "2", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+	cl_must_pass(odb_source_write_object(&source->base, "3", 1,
+					     OBJ_BLOB, &written_oid, NULL, 0));
+
+	cl_must_pass(odb_source_count_objects(&source->base, 0, &count));
+	cl_assert_equal_u(count, 3);
+
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__find_abbrev_len(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct object_id oid1, oid2;
+	unsigned abbrev_len;
+
+	/*
+	 * The two blobs we're about to write share the first 10 hex characters
+	 * of their object IDs ("a09f43dc45"), so at least 11 characters are
+	 * needed to tell them apart:
+	 *
+	 *   "368317" -> a09f43dc4562d45115583f5094640ae237df55f7
+	 *   "514796" -> a09f43dc45fef837235eb7e6b1a6ca5e169a3981
+	 *
+	 * With only one blob written we expect a length of 4.
+	 */
+	cl_must_pass(odb_source_write_object(&source->base, "368317", strlen("368317"),
+					     OBJ_BLOB, &oid1, NULL, 0));
+	cl_must_pass(odb_source_find_abbrev_len(&source->base, &oid1, 4,
+						&abbrev_len));
+	cl_assert_equal_u(abbrev_len, 4);
+
+	/*
+	 * With both objects present, the shared 10-character prefix means we
+	 * need at least 11 characters to uniquely identify either object.
+	 */
+	cl_must_pass(odb_source_write_object(&source->base, "514796", strlen("514796"),
+					     OBJ_BLOB, &oid2, NULL, 0));
+	cl_must_pass(odb_source_find_abbrev_len(&source->base, &oid1, 4,
+						&abbrev_len));
+	cl_assert_equal_u(abbrev_len, 11);
+
+	odb_source_free(&source->base);
+}
+
+void test_odb_inmemory__freshen_object(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	struct object_id written_oid;
+	struct object_id oid;
+	const char *end;
+
+	cl_must_pass(parse_oid_hex_algop(RANDOM_OID, &oid, &end, repo.hash_algo));
+	cl_assert_equal_i(odb_source_freshen_object(&source->base, &oid), 0);
+
+	cl_must_pass(odb_source_write_object(&source->base, "foobar",
+					     strlen("foobar"), OBJ_BLOB,
+					     &written_oid, NULL, 0));
+	cl_assert_equal_i(odb_source_freshen_object(&source->base,
+						    &written_oid), 1);
+
+	odb_source_free(&source->base);
+}
+
+struct membuf_write_stream {
+	struct odb_write_stream base;
+	const char *buf;
+	size_t offset;
+	size_t size;
+};
+
+static ssize_t membuf_write_stream_read(struct odb_write_stream *stream,
+					unsigned char *buf, size_t len)
+{
+	struct membuf_write_stream *s = container_of(stream, struct membuf_write_stream, base);
+	size_t chunk_size = 2;
+
+	if (chunk_size > len)
+		chunk_size = len;
+	if (chunk_size > s->size - s->offset)
+		chunk_size = s->size - s->offset;
+
+	memcpy(buf, s->buf + s->offset, chunk_size);
+
+	s->offset += chunk_size;
+	if (s->offset == s->size)
+		s->base.is_finished = 1;
+
+	return chunk_size;
+}
+
+void test_odb_inmemory__write_object_stream(void)
+{
+	struct odb_source_inmemory *source = odb_source_inmemory_new(odb);
+	const char data[] = "foobar";
+	struct membuf_write_stream stream = {
+		.base.read = membuf_write_stream_read,
+		.buf = data,
+		.size = strlen(data),
+	};
+	struct object_id written_oid;
+
+	cl_must_pass(odb_source_write_object_stream(&source->base, &stream.base,
+						    strlen(data), &written_oid));
+	cl_assert_equal_s(oid_to_hex(&written_oid), FOOBAR_OID);
+	cl_assert_object_info(source, &written_oid, OBJ_BLOB, "foobar");
+
+	odb_source_free(&source->base);
+}
diff --git a/t/unit-tests/u-oidtree.c b/t/unit-tests/u-oidtree.c
index d4d05c7dc3..f0d5ebb733 100644
--- a/t/unit-tests/u-oidtree.c
+++ b/t/unit-tests/u-oidtree.c
@@ -19,7 +19,7 @@ static int fill_tree_loc(struct oidtree *ot, const char *hexes[], size_t n)
 	for (size_t i = 0; i < n; i++) {
 		struct object_id oid;
 		cl_parse_any_oid(hexes[i], &oid);
-		oidtree_insert(ot, &oid);
+		oidtree_insert(ot, &oid, NULL);
 	}
 	return 0;
 }
@@ -38,9 +38,9 @@ struct expected_hex_iter {
 	const char *query;
 };
 
-static int check_each_cb(const struct object_id *oid, void *data)
+static int check_each_cb(const struct object_id *oid, void *node_data UNUSED, void *cb_data)
 {
-	struct expected_hex_iter *hex_iter = data;
+	struct expected_hex_iter *hex_iter = cb_data;
 	struct object_id expected;
 
 	cl_assert(hex_iter->i < hex_iter->expected_hexes.nr);
@@ -105,3 +105,23 @@ void test_oidtree__each(void)
 	check_each(&ot, "32100", "321", NULL);
 	check_each(&ot, "32", "320", "321", NULL);
 }
+
+void test_oidtree__insert_overwrites_data(void)
+{
+	struct object_id oid;
+	struct oidtree ot;
+	int a, b;
+
+	cl_parse_any_oid("1", &oid);
+
+	oidtree_init(&ot);
+
+	oidtree_insert(&ot, &oid, NULL);
+	cl_assert_equal_p(oidtree_get(&ot, &oid), NULL);
+	oidtree_insert(&ot, &oid, &a);
+	cl_assert_equal_p(oidtree_get(&ot, &oid), &a);
+	oidtree_insert(&ot, &oid, &b);
+	cl_assert_equal_p(oidtree_get(&ot, &oid), &b);
+
+	oidtree_clear(&ot);
+}